Slurs carry weight due to historical context. The n-word, at least in America, is dehumanizing of blacks and invokes years of slavery and oppression. Essentially, it comes down to dehumanizing connotation.
"Cracker" or "Redneck" don't carry anywhere near the same context. There's no period of white oppression, at least in America, that the words could invoke. There's no real weight.
When you're using a slur, you're essentially saying this: "You are not my equal. You are beneath me. You are not a human worthy of respect or value. I am rallying behind this word that unifies me with like-minded individuals, and together we hate you." There has never been an anti-white movement with enough power to conjure up a similar message in a single word.